1. Field of the Invention
This invention relates generally to a medication infusion device for administering fluid to patients and more particularly to an improved, ambulatory infusion device with a disposable administration set which is inexpensive to manufacture, convenient to operate and which ensures fluid delivery at a consistent and uniform rate.
2. Description of the Prior Art
As a result of the ongoing need for improved health care, there is a continuous effort with regard to administering intravenous fluid to patients. As is well known, medication dispensers and infusion devices are used for infusion of predetermined amounts of medication into the body of a patient. Various types of medication dispensers employing different techniques for a variety of applications are known to exist. Some existing infusion devices utilize a series of diaphragms which roll up and down inside cylinders in response to pistons located within the cylinders. Such devices, for example, as disclosed in U.S. Pat. Nos. 4,236,880, 4,277,226, 4,391,600 and 4,410,322 are characterized by relatively complicated operational procedures and numerous manufacturing and maintenance adjustments to ensure proper operation of the loading and valving functions.
Other existing infusion devices employ a finger type pump unit having fingers which are moved in predetermined sequence to squeeze a feeding tube to infuse predetermined amounts of medication continuously and cyclically into a patient. Such a prior art device is disclosed in U.S. Pat. No. 4,479,797.
In many cases it is of critical importance to provide precisely controlled and consistent flow rates of intravenous fluid to patients. This need for more controlled IV flow rates is only partially fulfilled by the above-mentioned displacement pumps. The finger type displacement pump unit as disclosed in U.S. Pat. No. 4,479,797 includes a plurality of elongate fingers which are depressed by an equal number of associated cams rotating about a shaft which is driven by a motor. A feeding tube, when placed between the fingers, is squeezed by the fingers to infuse liquid medication into a human body. The elongate fingers comprise a central finger and two side fingers arranged on either side. In such a device the side fingers completely squeeze or collapse the feeding tube at the designated point of contact on either side, and the central finger is shaped for squeezing the feeding tube and pumping medication over the length of tube between the two points, providing a pulsatile flow of fluid. During the time that the downstream side finger completely squeezes the feeding tube, while the pumping portion of the tube is filling from an upstream supply, the flow of fluid to the patient is completely obstructed.
Completely obstructing the flow of medication for periods of time and providing pulsatile flow is often a clinically unsatisfactory and undesirable condition since a patient is deprived of medication for periods of time. Additionally, fluid is delivered at non-uniform rates. Sometimes, the possibility exists at low flow rates for a catheter to develop a blood clot due to lack of flow. This may also result in an occluded tubing, so that the patient may be deprived of needed medication for several hours. This condition is especially likely in a home environment which seldom has around-the-clock clinical staffing for monitoring patients. Thus, it is desirable to have an infusion device which delivers fluid to a patient at a constant and uniform rate.
Some prior art devices, in an attempt to provide non pulsatile flow of fluid, incorporate additional pulsation correction cams and cam followers. The correction cams are designed to even out overall flow from the pump through simultaneous pumping actions of multiple cams. This solution is undesirable in view of the numerous parts involved. Infusion devices utilizing piston valves and cylinders are also known to provide uniform flow; however, they do so at the expense of crude valving techniques and complex mechanisms. Thus it is desirable to have an infusion device utilizing a relatively simple technique of providing non-pulsatile flow.
As is well known, disposable equipment for medical applications is desirable so as to maintain a germ-free environment to prevent the transfer of infection especially where cost prohibits cleaning and sterilization after each use. Prior art devices employing a series of diaphragms rolling within cylinders have utilized disposable diaphragms. The disposable diaphragms, however, are flimsy and thus require a very complex loading mechanism. Prior art devices having finger type pump units, for example, as disclosed in U.S. Pat. No. 3,658,445, have only a disposable tube assembly. This has limited advantages, since the cam followers, which are not a part of the disposable assembly, are not renewed with each replacement. U.S. Pat. No. 4,479,797 discloses other disposable elements. Since the cam followers are repeatedly flexed and fatigued as a result of being depressed by the rotating cams, it is desirable to have them renewed with every new installation to ensure proper functioning.
Thus, it is desirable to have a disposable administration set, including the pumping tube itself, a base plate against which the tube is depressed, and the cam followers. With such an arrangement, the system operates reliably and accurately over a longer period of time because its major pumping components are replaced with each use. Another important consideration for disposable elements is cost, since clinical application of disposable administration sets requires that the administration sets be regularly replaced. Typically, such sets are replaced every 24 to 48 hours, and seldom remain in use longer than one week. This frequent replacement interval should ideally be fulfilled by an inexpensively molded, disposable, mechanism which would normally not last the years of service life expected from the pump itself.
Furthermore, it is desireable to have a disposable administration set which is easy to load and unload to minimize operator errors. These factors can be very important in a clinical situation when a few extra seconds may be critical to a patient""s life. Typically, prior art devices require several steps to accomplish the task of loading and unloading. Additionally, it is beneficial to pinch off the delivery tube and obstruct fluid flow prior to loading or unloading the administration set.
It is also desireable to have an efficient but inexpensive occlusion detection system. Some prior art devices incorporate a pressure transducer and diaphragm assembly to monitor fluid pressure as an indication of occlusion. Such an occlusion detection technique is undesirable in view of the complexities and cost involved. Prior art devices utilizing pistons and cylinders detect occlusion by utilizing a switch mounted within the cylinder. When the pressure reaches a certain value the rotating diaphragm causes the switch to be depressed. The switch may also be mounted against the tubing such that the switch is activated when the tubing expands as a result of increased pressure due to an occlusion.
Briefly stated, the present invention is an improved, ambulatory infusion device having a disposable administration set which is inexpensive to manufacture and a pumping mechanism which provides a consistent and uniform fluid flow.
The infusion device comprises an administration set having a plurality of cam followers which are depressed in a predetermined sequence by a plurality of associated rotating cams. The cam followers squeeze a delivery tube to dispense fluid which is intravenously infused into a patient""s body. The rotating cams are mounted to an axle shaft driven by a motor. The disposable administration set further comprises a plate assembly to which the cam followers are attached. The cam followers force the delivery tube against the plate assembly to pinch off the delivery tube or force fluid to flow through it. In the preferred embodiment of the present invention the administration set is advantageously disposable and inexpensive to manufacture. In accordance with one feature of the present invention the cam followers are all molded together as one piece. Alternatively, the cam followers may also be molded as one piece with the plate assembly, provided that a hinge is molded to connect the cam followers with the plate assembly. This provides a cost effective assembling technique which allows the cam followers and plate to be replaced regularly at minimal cost.
Since the cam followers are instrumental in controlling the amount of fluid dispensed, the thickness of the cam followers is a critical dimension which directly affects the degree of tube compression, and thus the cross sectional area of the delivery tube. Because some variations in thickness can be expected from one molded part to the next due to normal molding process variations, the invention utilizes gap correction spacers which counteract these thickness variations. The gap correction spacers fit between the plate and the support for the rotating cam, and thus adjust the distance between the cam and the plate. If the cam follower assembly varies in thickness, for example due to change in the pressure of an injection molding machine, both the cam followers and the gap correction spacers vary by the same amount, because they are molded as a single unit. Since an increase in thickness of the gap correction spacers results in an increase in the delivery tube gap, while an increase in the thickness of the cam followers results in a decrease in the delivery tube gap, the net effect is no change in the delivery tube gap. It is this correction technique that allows the followers to be injection molded without sacrificing the accuracy of the fluid delivery.
An additional feature of the invention allows the use of a low cost molded pressure plate assembly. Springs are used to force the face of the tubing retainer plate against the gap correction spacers. These springs, of course, must be stiff enough to be unyielding as the cams squeeze the delivery tube. By floating the plate on such springs, changes in thickness from plate to plate due to molding variations do not change the tubing gap from part to part. This is another important part of the invention which allows an easily loaded and inexpensive disposable administration set to be used without sacrificing performance or accuracy.
The plate referencing system and the gap correction spacers described above are adapted to increase the accuracy of the delivery system. In cooperation with the gap correction spacers, a channel in the pumping system receives the retainer plate and the spacers. The springs mentioned above are mounted in this channel and force the disposable assembly against a reference shoulder within the channel. This shoulder allows an easily manufactured dimensional precision to accurately define the tubing gap. The critical dimensions are the cam radius itself and the distance between the cam axis and the plate referencing shoulder. These two dimensions can be precisely controlled in the manufacturing process and they will not change significantly with time or usage.
Another feature of the present invention exists in the ability to deliver fluid at a consistent and uniform rate. The pumping mechanism includes an axle shaft and a plurality of cams mounted thereto. As the axle shaft rotates, the cams force the cam followers to squeeze the tubing and thereby displace a certain volume of fluid which is then forced out of the pump. The cams are structurally adapted such the each incremental angle of revolution displaces the same amount of fluid. This is facilitated by a non-linear cam design which provides a non-linear change in the delivery tube gap. The change in cross sectional area of the delivery tube caused by a given change in the gap depends on the tube gap at the start of the change. The cam non-linearity is designed to correlate with this change.
The present invention utilizes two pumping cams and two pumping cam followers, which function such that, at any point in time, one of the two pumping cams is always pumping. The two pumping cams comprise a primary pumping cam associated with an upstream segment of the delivery tube and a secondary pumping cam associated with a downstream segment of the delivery tube. The primary pumping cam is wider than the secondary pumping cam, so that it can displace sufficient fluid during its pumping stroke to deliver fluid external to the pump and at the same time deliver fluid to the section of the tubing beneath the secondary pumping cam to allow it to fill. The secondary pumping cam is narrower, since it only needs to deliver fluid external to the pump. The present invention additionally utilizes pinching cams and pinching cam followers, which open and close the delivery tube to allow the pumping action to function properly. The pinching cams comprise an inlet pinching cam associated with the upstream segment of the tube and an outlet pinching cam associated with the downstream segment of the tube. Thus the pumping cam followers, acted upon by the pumping cams, control the rate of fluid flow, while the pinching cam followers acted upon by the pinching cams, operate as valves for the pump. Such a configuration allows one segment of the delivery tube to fill with fluid while another segment of the delivery tube is pumping, thus providing a continuous and uniform fluid flow.
In still another feature of the present invention the disposable administration set of the infusion device is less prone to operator loading errors. This is accomplished through a reduced number of required operations and a reduction in the complexity of the operations. This is facilitated by providing channels extending along the length of the walls of a housing structure of the infusion device. These channels slidingly receive the disposable administration set in a simple, single insertion step. Additionally, since the disposable administration set includes the delivery tube retainer plate and cam followers, the position of the delivery tube relative to the tubing retainer plate and cam followers is established in a manufacturing operation which can be closely controlled. Assemblers are not under the stress of a clinical situation and they specialize in the proper assembly of the disposable administration set. Good manufacturing procedures provide additional checking systems to insure that the tubing is properly loaded and that the administration set properly assembled. These practices are not possible in a clinical environment.
The pump of the infusion device is non-functional when loaded with an alien disposable administration set. Alien administration sets may have characteristics which are not suitable for safe operation of the fluid delivery system, and may operate outside of specified tolerance limits. The pump will not operate with a standard piece of delivery tube, because it requires an administration set with cam followers and a pressure plate.
Another feature of the present invention is a provision for detecting an occlusion in the downstream fluid path. The cams which squeeze the tubing are rotated by a DC motor having a predictable torque-to-current transfer function. By monitoring the current to the motor, the amount of torque required to maintain a desired camshaft velocity can be measured. With knowledge of the motor torque required to advance the cam at each position along its rotation, against a normal fluid back-pressure, and comparing this torque with the actual measured torque, higher-than-normal pressure in the delivery tube can be sensed. If desired, a calculation can be made to determine the torque normally required to rotate the cam. This torque calculation takes into account the pressure exerted by the tube against the cam face, the effective radius of the cam-follower contact point, and the coefficient of friction of the cam-to-follower contact for each cam rotational position. This calculation is undertaken for each cam, to yield a total accumulated torque value. This total torque profile is stored in a memory device to be read out and compared with the actual torque for monitoring abnormally high pumping pressures.
Alternatively, the memory may be loaded with actual D.C. current data read during a previous operation of the pump with normal pressure. This profile is unique to the particular mechanism and instrument in which it is installed. The profile is a current waveform sampled at specific angles of revolution of the pumping system. The waveform is sampled and stored during the actual operation of the pump under controlled conditions, i.e. specific output pressure, specific temperature, etc. During later operation of the pump, these stored values are periodically compared to the actual operating current and an alarm is signalled if the difference between these two values exceeds a specified tolerance.
The control system also has access to the current operating temperature which may be used to adjust the allowable tolerance for changes in the operating temperature. The control system may also have access to data stored on the disposable administration set which indicates particular information related to manufacturing variances, such as coefficient of friction, stroke volume, tubing gap, tubing wall thickness and diameter, etc. This information may also be used to determine the appropriate alarm point. The information may be stored on the disposable administration set so that it may be read by the instrument during the sliding operation of loading the set.
The present invention uses a minimal number of parts and dissipates a minimal amount of energy. The disposable concept which includes the cam followers and the pressure plate allows for a high precision pump without complicated assembly or loading mechanisms. The set loading and retaining channels allow precise positioning of the tubing, followers, and pressure plate without any adjustments or complicated, bulky, or expensive mechanisms. The disposable administration set results in an overall fluid delivery system which is small, lightweight, and ambulatory.
The design of the disposable administration set in combination with the plate referencing channels allows a sliding operation in order to load the set. This sliding operation allows for the transfer of information from the disposable administration set to the instrument from fixed sensors. These sensors may be optical, magnetic, or some other technology. The preferred embodiment uses optical sensors to read optically coded information from a label on the administration set. This capability permits the instrument to be programmed from the information included on the label. The instrument operator is thus free from programming tasks, which would be difficult in a clinical environment. Programming information can be added to the administration set during the preparation or prescription of the medication to be delivered. The unique sliding operation makes this programming simple and cost effective in the present invention.